A dump truck is provided with a variety of brakes such as a foot brake, a retarder brake, a parking brake, an exhaust brake, and an emergency brake.
The front brake of a dump truck is constituted, for example, by a caliper brake. Furthermore, the rear brake is constituted by a wet multiplate brake and forcibly cooled by oil for cooling (cooling oil). A brake cooling circuit of the rear brake is so configured that the cooling oil circulates between the rear brake and the water-cooled oil cooler. The configuration is also such that cooling water of the oil cooler circulates between the oil cooler and the radiator. Hydraulic oil is supplied from a hydraulic pump that is driven by an engine to a hydraulic motor, and the rotation of the hydraulic motor drives the hydraulically driven fan for cooling the radiator.
The cooling oil in the brake cooling circuit absorbs the heat generated from the brake disk in the rear brake, and the absorbed amount of heat is dissipated by heat exchange with the cooling water in the water-cooled oil cooler. The cooling water is cooled in the radiator by a flow of air blown by the hydraulically driven cooling fan, and the heat of the cooling water is emitted into the atmosphere. Alternatively, this heat is directly dissipated into the atmosphere in an air-cooled oil cooler.
In the foot brake, the front brake and rear brake are actuated in accordance with the depression of a pedal.
In the retarder brake, only the rear brake is actuated in accordance with the operation of a retarder control lever.
Where the foot brake is continuously or repeatedly used when a dump truck descends a long slope, while carrying a heavy load, it is possible that the front brake will be overheated, the friction coefficient will be decreased, a fade effect will occur, and a brake efficiency will be degraded. Therefore, during downhill movement, the retarder brake is applied by operating the retarder control lever. With the retarder brake, braking is performed only with the rear brake that is forcibly cooled by cooling oil, the heat generated by the disk of the rear brake is absorbed by the cooling oil, and the occurrence of the fade effect or the like is inhibited.
The capacity of the retarder is typically determined by the cooling capacity of the cooling oil that is forcibly circulated. For this reason, the cooling capacity of the cooling oil has to be increased in order to increase the retarder capacity.
(Conventional Art 1 that has been Used)
In the conventional control of rotation speed of the aforementioned hydraulically driven cooling fan that has been heretofore performed in dump trucks, the cooling oil temperature and engine cooling water temperature are detected by respective sensors installed on the vehicle, the fan rotation speed is adjusted so that the rotation speed of the hydraulically driven cooling fan increases as the detected cooling oil temperature and cooling water temperature rise, the cooling capacity of the cooling oil rises, and the cooling oil is prevented from overheating.
The below-described patent documents relate to rotation speed control of a hydraulically driven cooling fan in the field of vehicles other than a dump truck, for example, a hydraulic shovel and a bulldozer.
(Conventional Art 2 Disclosed in a Patent Document)
Patent Document 1 describes an invention according to which in a configuration provided with an oil cooler that cools hydraulic oil for driving a working machine and a hydraulically driven cooling fan that cools the hydraulic oil that passes through the oil cooler, the temperature of the engine cooling water, temperature of the hydraulic oil, and rotation speed of the engine are detected by respective sensors, and the rotation speed of the hydraulically driven cooling fan is controlled correspondingly to the detected values of the sensors.
(Conventional Art 3 Disclosed in a Patent Document)
Patent Document 2 describes an invention according to which in a configuration provided with an oil cooler that cools hydraulic oil for driving a working machine and a hydraulically driven cooling fan that cools the hydraulic oil that passes through the oil cooler, the temperature of the engine cooling water, temperature of the hydraulic oil, and temperature of an intake air are detected by respective sensors, and the rotation speed of the hydraulically driven cooling fan is controlled correspondingly to the detected values of the sensors.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-182535
Patent Document 2: Japanese Patent Application Laid-open No. 2000-110779